Sensor and method of manufacturing the same

ABSTRACT

A sensor comprising a semiconductor film having a plurality of mesopores and containing an oxide, and electrodes electrically connected to the semiconductor film, wherein at least part of surfaces in the mesopores is coated with an organic material.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a sensor for detecting a substance thatis adsorbed to and/or desorbed from the surface of semiconductor and amethod of manufacturing the same. This invention can be applied to a gassensor and a bio sensor.

2. Related Background Art

Gas sensors and bio sensors of various types have been proposed todetect the presence or absence of a substance and its concentration.

Among such sensors, semiconductor type sensors are adapted to detect asubstance, utilizing a metal oxide semiconductor having a property ofchanging its resistance in response to adsorption or desorption of thesubstance. Thus, semiconductor type sensors find applications in gasescape alarms and so on. Oxide semiconductors, tin oxide in particularare popularly used for gas sensors. One of the challenges to existinggas sensor elements is selective detection of a gas as an object ofdetection.

Due to absorption and/or desorption of molecules that are adsorbed tothe surface of a metal oxide semiconductor, the width of the depletionlayer that is present on the surface of the metal oxide changes as afunction of the adsorption or desorption. Thus, metal oxidesemiconductor elements that are used in gas sensor elements are mostlydevised to detect the change in the electric resistance of thesemiconductor element. In the case of tin oxide, for instance, oxygenmolecules adsorbed to the surface of tin oxide are removed as a resultof a chemical reaction (combustion) of a gas and the oxygen adsorbed tothe surface of tin oxide, and consequently the width of the depletionlayer is reduced to by turn lower the electric resistance of the tinoxide.

Due to the above-described principle of operation, it is difficult todetect only a target gas in an atmosphere where gases of a plurality ofdifferent types that can change the width of the depletion layer coexistwhen a single metal oxide semiconductor is used.

Therefore, it is an object of the present invention to provide a sensorthat can detect only a target gas to be detected.

SUMMARY OF THE INVENTION

In an aspect at the present invention, the above object is achieved byproviding a sensor comprising: a semiconductor film having a pluralityof mesopores and containing an oxide; and electrodes electricallyconnected to the semiconductor film; at least part of surfaces in themesopores being coated with an organic material.

In another aspect of the present invention, there is provided a sensorcomprising: a semiconductor film having a plurality of mesopores andcontaining an oxide; and electrodes electrically connected to thesemiconductor film; at least part of surfaces in the mesopores beingcoated with an oxide different from said oxide.

In still another aspect of the present invention, there is provided asensor comprising: a semiconductor film having a plurality of mesoporesand containing a tin oxide; and electrodes electrically connected to thesemiconductor film; particles of an inorganic material being held in themesopores.

In still another aspect of the present invention, there is provided amethod of manufacturing a sensor comprising: preparing a reactionsolution containing a metal compound and a surfactant and applying thereaction solution onto a substrate; holding the substrate with thereaction solution applied thereto in a steam-containing atmosphere andforming a film containing a metal oxide and the surfactant on thesubstrate; removing the surfactant from the film and producing a filmhaving a plurality of mesopores; and causing the mesopores to holdparticles of an inorganic material.

In still another aspect of the present invention, there is provided amethod of manufacturing a sensor comprising: preparing a reactionsolution containing a metal compound and a surfactant and applying thereaction solution onto a substrate; holding the substrate with thereaction solution applied thereto in a steam-containing atmosphere andforming a film containing a metal oxide and the surfactant on thesubstrate; removing the surfactant from the film and producing a filmhaving a plurality of mesopores; and coating at least part of surfacesin the mesopores with an organic material.

In still another aspect of the present invention, there is provided amethod of manufacturing a sensor comprising: preparing a reactionsolution containing a metal compound and a surfactant and applying thereaction solution onto a substrate; holding the substrate with thereaction solution applied thereto in a steam-containing atmosphere andforming a film containing a metal oxide and the surfactant on thesubstrate; removing the surfactant from the film and producing a filmhaving a plurality of mesopores; and coating at least part of surfacesin the mesopores with an inorganic oxide.

Thus, the present invention can provide a sensor that can selectivelydetect molecules of a specific gas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view of a sensor according to theinvention, showing its configuration;

FIG. 2 is a schematic cross sectional view of the sensor of FIG. 1;

FIG. 3 is an enlarged schematic view of a mesopore in FIG. 2;

FIG. 4 is an enlarged schematic view of a mesopore in FIG. 2;

FIG. 5 is an enlarged schematic view of a mesopore in FIG. 2;

FIGS. 6A, 6B, 6C and 6D are schematic illustrations of particles of aninorganic material;

FIG. 7 is a schematic cross sectional view of an embodiment of sensoraccording to the invention;

FIG. 8 is a schematic cross sectional view of another embodiment ofsensor according to the invention;

FIG. 9 is a schematic illustration of an arrangement of mesopores;

FIG. 10 is a flowchart of a method of manufacturing a sensor accordingto the invention;

FIG. 11 is a schematic illustration of an apparatus for evaluating thecharacteristics of a sensor according to the invention;

FIG. 12 is a graph illustrating the changes in the electric resistanceobserved when a plurality of gases is detected by means of a sensoraccording to the invention;

FIG. 13 is a graph illustrating the changes in the electric resistanceobserved when a plurality of gases is detected by means of a sensorprepared for the purpose of comparison with a sensor according to theinvention;

FIG. 14 is a graph illustrating the changes in the electric resistanceobserved when a plurality of gases are detected by means of a sensoraccording to the invention;

FIG. 15 is a graph illustrating the changes in the electric resistanceobserved when a plurality of gases are detected by means of a sensorelement prepared for the purpose of comparison with a sensor accordingto the invention;

FIG. 16 is a graph illustrating the changes with time in the electricresistance observed when a plurality of gases are detected by means of asensor according to the invention; and

FIG. 17 is a graph illustrating the changes with time in the electricresistance observed when a plurality of gases are detected by means of aknown sensor for the purpose of comparison.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, the present invention will be described in detail. Firstly, thepresent invention will by summarily described by referring to FIGS. 1,2, 3, 4 and 5.

FIG. 1 is a schematic perspective view of a sensor according to theinvention, showing its configuration. A pair of electrodes 12, 12 isarranged on a substrate 13 and a porous oxide semiconductor film 11 isformed thereon as a sensing section. FIG. 2 is a schematic crosssectional view of the sensor of FIG. 1 taken along line 2-2. A pair ofelectrodes 12, 12 and a porous oxide semiconductor film 11 are formed onthe substrate 13. The porous oxide semiconductor film 11 has a pluralityof mesopores 14. While mesopores will be described in detailhereinafter, FIGS. 3, 4 and 5 provide enlarged views of mesopores. FIG.3 is an enlarged schematic view of a mesopore, showing that the surfacein the mesopore is coated with an organic material. FIG. 4 is anotherenlarged schematic view of a mesopore, showing that the surface in themesopore is coated with an oxide different from the oxide contained inthe porous oxide semiconductor film 11. FIG. 5 is still another enlargedschematic view of a mesopore, showing that particles of an inorganicmaterial are held in the mesopore. A sensor according to the presentinvention is so devised as to be capable of selectively detecting gasmolecules by coating the surfaces in the mesopores or causing thesurfaces in the mesopores to hold particles of an inorganic material.

A “mesopore” is defined to be a pore with a pore size between 2 nm and50 nm according to the classification of IUPAC. A pore with a pore sizeless than 2 nm is defined to be a micropore, while a pore with a poresize greater than 50 nm is defined to be a macropore.

The specific surface area is smaller in a structure having macroporeswhose sizes are larger than those of mesopores than in a structurehaving a plurality of mesopores. A structure having a smaller specificsurface area can hold a lesser amount of particles of an inorganicmaterial or a lesser amount of an oxide of an organic material in thepores. Additionally, such a structure can adsorb a lesser amount of gasmolecules to be detected. On the other hand, in a structure havingmicropores whose sizes are smaller than those of mesopores, it can bedifficult to introduce particles of an inorganic material into themicropores and quickly adsorb gas molecules to or desorb gas moleculesfrom the surfaces of the micropores.

Therefore, a structure having mesopores can most suitably be used for agas sensor or a bio sensor to detect small molecules.

A “porous oxide semiconductor film” is preferably a continuous film.Such a film can show a higher efficiency of utilization and a higherresponse speed than a porous oxide semiconductor film 11 that is formedas an aggregate of particles when used for a sensor. When the porousoxide semiconductor film 11 is an aggregate of particles in a structure,the structure shows a smaller specific surface area than a structurehaving mesopores. Then, the efficiency of utilization of the element maynot become high and the response speed may be low when it is used for asensor.

The “oxide contained in the porous oxide semiconductor film” ispreferably a metal oxide that shows characteristics of semiconductor.Examples of metal oxides that show such characteristics include tinoxide (SnO₂), zinc oxide (ZnO), niobium oxide (Nb₂O₅) and tungsten oxide(WO₃), of which tin oxide (SnO₂) can preferably be used for the purposeof the present invention.

Any “organic material” that can selectively catch the detectionsubstance, or the detection substances, and change the electricresistance of the metal oxide may be used. Particularly, organicmaterials containing one or more than one Lewis bases are preferablecandidates. Such organic materials have any of the functional groups andbonds listed below as examples.

Examples of functional groups include halogens, alcohols, amines,nitrites, nitros, sulfides, sulfoxides, sulfones, thiols, carbonyls,aldehydes, ketones, carbonic acids, amides, carbonic acid chlorides,carbonic acid anhydride and organic carbonic acids.

Examples of bonds include ether bonds, ester bonds and amide bonds.

For the purpose of the present invention, the “oxide different from theoxide contained in the porous oxide semiconductor film 11” may be anyoxide that can selectively separate a specific gas or reacts with aspecific gas and can change the electric resistance of the porous oxidesemiconductor film. Examples of such oxides include silicon oxide.Silicon oxide selectively allows a gas having a small molecular weightto pass through it. More specifically, silicon oxide selectively allowshydrogen to pass through it so that it can be used for a hydrogensensor.

“Particles of an inorganic material” may be those of any material thatcan separate or decompose the detection substance from gas or liquidcontaining the detection substance by means of a catalytic effect.Examples of inorganic materials that can be used for the purpose of thepresent invention include palladium (Pd), platinum (Pt), ruthenium (Ru),silver (Ag), cobalt (Co), gold (Au), nickel (Ni), copper (Cu), manganese(Mn), iron (Fe), chromium (Cr) and vanadium (V). Particularly, from theviewpoint of hydrogen gas, palladium (Pd) and platinum (Pt) arepreferable because they show an excellent selectivity relative tohydrogen gas.

Particles of any metal oxide having a similar effect may also be used.Examples of such metal oxides include copper oxide (II) (CuO), nickeloxide (NiO) and cobalt oxide (CoO), of which copper oxide (II) (CuO) maypreferably be used for a sensor for detecting hydrogen sulfide becauseit is highly selective relative to hydrogen sulfide.

Particles of an inorganic material may show any profile so long as theycan be introduced into pores. For examples, they may show any of theprofiles illustrated in FIGS. 6A, 6B, 6C and 6D that include spherical(FIG. 6A), wire-shaped (FIG. 6B), rod-shaped (FIG. 6C) and tubular (FIG.D).

Particles of an inorganic material may have any size so long as they canbe held in pores to prevent gas from diffusing into the pores. Ifparticles of an inorganic material are spherical, it is preferable thattheir sizes are smaller than the sizes of the pores.

“Electrodes” may be comb-like electrodes as shown in FIG. 1 that may berealized by bonding a plurality of electrodes. For example, anarrangement where a plurality of electrodes are formed on the porousoxide semiconductor film as shown in FIG. 7 or an arrangement whereelectrodes are formed on and under the porous oxide semiconductor filmas shown in FIG. 8 may be used for the purpose of the present invention.The electrodes are connected to an electric circuit to measure thechange in the electric resistance of the porous material and determineif there is the detection substance, or the target substance to bedetected.

For the purpose of improving the sensitivity of a sensor, a heaterand/or microcrystals may be used and the arrangement and the pore sizesof mesopores (fine pores) may be defined in a manner as described below.

1. Heater

A heater may be arranged in order to accelerate theadsorption/desorption reaction of the detection substance and heat thesubstrate and the porous oxide semiconductor film. On a cold site, it isconvenient to set the temperature of the sensor optimally for thereaction with gas. A heater is preferably arranged at a position whereit is held in contact with the porous oxide semiconductor film, althoughsome other layer may be disposed between the porous oxide semiconductorfilm and the heater.

2. Arrangement and Pore Size of Mesopores

FIG. 9 shows mesopores that are arranged to show a two-dimensionalhexagonal structure. However, the arrangement of fine pores is notlimited to such a structure. For example, they may alternatively bearranged to show a distorted two-dimensional hexagonal structure, acubic structure or a three-dimensional hexagonal structure. The finepores may have a unique size or different sizes that do not show anyclear periodicity.

The porous oxide semiconductor film 11 preferably shows one or more thanone diffraction peaks in the angular region that corresponds to thestructural periodicity of not less than 1 nm in X-ray diffractionanalysis. This is because the pore arrays in the porous oxidesemiconductor film shows a regular structural periodicity, mesopores aredensely arranged to raise the specific surface area thereof in theporous oxide semiconductor film.

A technique for measuring adsorption isotherms of gas such as nitrogengas is generally used to evaluate the distribution of fine pores in aporous oxide semiconductor film 11 and the distribution of fine poresare computationally determined from the obtained adsorption isothermstypically by means of the Berret-Joyner-Halenda (BJH) analysis method.

Thus, the distribution of fine pores of a porous oxide semiconductorfilm 11 to be used for the purpose of the invention are determined bythe BJH method from the data obtained by observing the nitrogen gasadsorption of the fine pores. It is preferable that the distribution offine pore diameters shows a unique maximum value and not less than 60%of the diameters of the fine pores are found within a range not greaterthan 12 nm.

3. Microcrystals

For the purpose of the present invention, the pore walls of the porousoxide semiconductor film 11 preferably contain microcrystals. When, forexample, tin oxide is used, it was found as a result of a series ofintensive researches conducted by the inventors of the present inventionon the correlation between the particle diameters of tin oxide and thesensitivity of detecting the detection substance that an enhanced levelof sensitivity can be achieved when the diameters of micro-particles arenot greater than 10 nm, particularly when the diameters are not greaterthan 6 nm.

The diameters of microcrystals can be controlled by controlling theconditions of the processing step using steam and the step of removingthe surfactant, which will be described in greater detail hereinafter.

(Method of Manufacturing Sensor)

Now, a method of manufacturing a sensor will be described below.

FIG. 10 is a flowchart of a method of manufacturing a porous oxidesemiconductor film for the purpose of the present invention.

Referring to FIG. 10, Step A is a step of preparing a reaction solutioncontaining a metal compound and a surfactant and applying the reactionsolution onto a substrate. Step B is a step of holding the substrate, towhich the reaction solution has been applied, in a steam-containingatmosphere and forming a film containing the metal oxide and thesurfactant. Step C is a step of removing the surfactant from the filmand forming a plurality of mesopores. Step D1 is a step of coating atleast part of surfaces in the mesopores with an organic material or anoxide. Step D2 is a step of holding particles of an inorganic materialin the mesopores.

As a result of carrying out Steps A and B, a filmy precursor of a porousoxide semiconductor film that is made of an aggregate of the surfactantand has a region that subsequently makes mesopores is formed on thesubstrate.

Such a structure is prepared by the process that the surfactant producesa micelle on the substrate to operate as a mold for producing mesopores(fine pores) and the metal compound forms the walls of the fine pores.

As the precursor of the porous oxide semiconductor film is held in asteam-containing atmosphere in Step B, the regularity of the fine porestructure of the precursor of the porous oxide semiconductor film isimproved and, at the same time, the oxide semiconductor that isamorphous immediately after the application of the reaction solution isinduced to become crystallized by steam.

Then, the surfactant is removed and a porous oxide semiconductor film isproduced as a result of carrying out the Step C. Subsequently, thesensor becomes provided with the function of selectively detecting thedetection substance as a result of modifying the film surface and thepore surfaces of the porous oxide semiconductor film and holdingparticles of the inorganic material in the pores as a result of carryingout Steps D1 and D2.

Thereafter, electrodes may be formed on the porous oxide semiconductorfilm for the purpose of detecting the change in the electric resistanceof the porous oxide semiconductor film. The electrodes may be formed inany of the above-described steps, after Steps D1 and D2 or before Step Aso long as the electrodes can be connected to the porous oxidesemiconductor film.

Now, each of the above listed steps will be described in greater detailbelow.

(Step A: step of preparing a reaction solution containing a metalcompound and a surfactant and applying the reaction solution onto asubstrate)

In Step A, firstly a reaction solution containing a metal compound and asurfactant is prepared.

(A-1: Preparation of a Reaction Solution)

The metal compound is a material for preparing an oxide semiconductorand contains, for example, tin (Sn), zinc (Zn), tungsten (W) or niobium(Nb). Particularly, a metal compound containing tin is preferably used.

Now, the preparation of reaction solution will be described in terms ofa tin compound.

Tin compounds that can be used for the purpose of the present inventioninclude chlorides such as tin chloride (I) (SnCl₂) and tin chloride (II)(SnCl₄) and tin alkoxides such as tin isopropoxide and tin ethoxide,although tin compounds that can be used for the purpose of the presentinvention are not limited to those listed above.

The surfactant produces a micelle that operates as a mold for producingfine pores.

A non-ionic surfactant is preferably used for the surfactant.

Non-ionic surfactants containing ethylene oxide are particularlypreferable. Examples of such surfactants include the following:

tri-block-copolymers such as <HO(CH₂CH₂O)₂₀ (CH₂CH(CH₃)O)₇₀(CH₂CH₂O)₂₀H>

polyoxyethylene (10) dodecylether <C₁₂H₂₅(CH₂CH₂O)₂₀H>

polyoxyethylene (10) tetradecylether <C₁₄H₂₉(CH₂CH₂O)₁₀OH>

polyoxyethylene (10) hexadecylether <C₁₆H₃₃(CH₂CH₂O)₁₀OH>

polyoxyethylene (10) stearylether <C₁₈H₃₇ (CH₂CH₂O)₁₀OH>.

Of the above-listed surfactants, triblock copolymers are preferablyused.

While alcohols such as methanol and ethanol are preferable candidate ofsolvent, a mixed solvent of alcohol and water can also be used. Inshort, any solvent that is liquid and can dissolve the metal compoundand the surfactant may be used for the purpose of the present invention.Acid or the like may be appropriately added to the solvent as catalyst.

Thus, the reaction solution is prepared in a manner as described above.

(A-2 Application of the Reaction Solution)

The prepared reaction solution is then applied onto a substrate.

A substrate that is chemically stable relative to the reaction solutionand hence hardly chemically reacts with the reaction solution ispreferably used. Examples of such substrates include glass substrates,ceramic substrates, resin substrates and metal substrates. Of course,flexible film such as plastic film may also be used for the substrate.

The electrodes and the porous material can be connected with ease afterSteps B and C, which will be described hereinafter, by using a substrateon which electrodes are formed as shown in FIGS. 1 and 2.

Examples of effective techniques that can be used for applying thereaction solution onto the substrate easily in a short period of timeinclude the casting, dip coating and spin coating.

Techniques other than those listed above such as spray coating, which ishighly suitable for mass production, may also be used so long as suchtechniques can be used to effectively apply the reaction solution ontothe substrate.

Thus, the reaction solution is applied onto the substrate in theabove-described manner. Note, however, that it is preferable to dry thereaction solution (particularly solvent) on the substrate after Step Aand before Step B.

For example, after Step A, it is preferable to carry out a drying stepof drying the solvent in a temperature range between 25 and 50° C. and ahumidity range between 10 and 30% before moving to Step B.

As a result of such an additional step, the uniformity of the film willbe improved.

(Step B: Holding the substrate, to which the reaction solution has beenapplied, in a steam-containing atmosphere and forming a film containingthe metal oxide and the surfactant on the substrate)

Then, the substrate which the reaction solution has been applied to anddried is held in a steam-containing atmosphere and a precursor of aporous oxide semiconductor film is formed.

The relative humidity of the steam-containing atmosphere of Step B ispreferably not lower than 40% and not higher than 100% and thetemperature of the steam-containing atmosphere is preferably not higherthan 100° C.

However, conditions out of the above-defined ranges may be used so longas it is possible to form the target substance, which is a precursor ofa porous oxide semiconductor film.

As a result of carrying out this step, the continuity of the film isremarkably improved and, at the same timer the uniformity of mesoporesin the porous oxide semiconductor and hence the structural regularity ofthe porous oxide semiconductor are also improved.

During Step B of holding the substrate, to which the reaction solutionis applied in a steam-containing atmosphere, the crystallization of theoxide semiconductor progresses.

The duration of the above process using steam may be appropriatelydetermined according to the level of crystallinity to be achieved.

(Step C: Removing the surfactant from the film and forming a pluralityof mesopores)

While a number of techniques may be available for removing thesurfactant, a baking process of applying heat to the surfactant todecompose and remove it is preferable because it is simple and providesan advantage of accelerating the crystallization of the oxidesemiconductor present on the fine pore walls.

However, it should be noted that, when the baking temperature is high,the crystallization of the oxide semiconductor progresses quickly butthe pore structure may tend to become irregular.

Therefore, when removing the surfactant by means of a baking process, itis necessary to select an optimum baking temperature level so that theregularity of the pore structure may be maintained.

When the material of the substrate can be deformed in high temperaturesprobably because the substrate is made of a plastic material and henceit is difficult to conduct a baking process, a technique of extractingthe surfactant by means of a supercritical fluid or a solvent mayfeasibly be used.

When the surfactant is extracted by means of a supercritical fluid or asolvent, it is possible to hold the density of hydroxyl groups on thepore surfaces to a high level after removing the surfactant. As aresult, it is possible to improve the density of the modification usingthe organic material.

The surfactant can be decomposed and removed by means of othertechniques such as irradiation of UV rays and oxidation/decomposition byozone. For the purpose of the present invention, techniques that can beused for decomposing and removing the surfactant are not limited to theabove listed ones and any appropriate method may be used so long as itcan maintain the pore structure of the porous oxide semiconductor film.

(Step D1: Coating at least part of surfaces in the mesopores with anorganic material or an oxide)

In this step, the surfaces of the pores and the film surface of theporous oxide semiconductor film are modified by means of a substancedifferent from that of the porous oxide semiconductor film.

Substances that can be used for modifying the surfaces of the fine poresinclude organic materials and inorganic materials.

Thus, organic materials and inorganic materials will be describedseparately.

1. When the substance for modifying the pore surfaces is an organicmaterial

While any appropriate method may be used to securely bond an organicmaterial to the surface of the porous oxide semiconductor film 11 bymeans of covalent bonds, a method of producing covalent bonds by meansof a silane coupling agent may preferably be used for the purpose of thepresent invention. This is because the organic material can be bonded tothe surface of an oxide semiconductor with ease when a silane couplingagent is used. After depositing the organic material on the surfaces ofthe pores of the porous oxide semiconductor film 11 by means of a silanecoupling agent, the functional groups of the organic material may bemodified and/or replaced by other functional groups by way of a chemicalreaction.

2. When the substance for modifying the pore surfaces is silicone oxide

While any appropriate method may be used to modify the surfaces of thefine pores by means of silicon oxide so long as the deposited siliconoxide does not clog the fine pores, it is preferable to bond a siliconcompound to the surfaces of the fine pores by means of a silane couplingagent and subsequently transform the silicon compound into silicon oxideby means of a heat treatment in an oxidizing atmosphere.

As a result of the above-described step, the surfaces of the fine porescan be coated relatively uniformly with silicon oxide.

(Step D2: Holding particles of an inorganic material in the mesopores)

In this step, particles of an inorganic material are held in the insideof the pores of the porous oxide semiconductor film.

Any appropriate method may be used for introducing particles of such amaterial into the inside of the pores so long as they do notsignificantly change the pore structure of the porous oxidesemiconductor film and remarkably reduce their specific surface areas.

When metal particles are to be held in the pores of the porous oxidesemiconductor film, it is preferable to introduce a metal compound thatis dissolved in a solution such as aqueous solution into the pores ofthe porous oxide semiconductor film and produce particles of metal inthe pores by reducing the metal compounds.

If, for example, palladium is introduced, a solution containing apalladium compound is firstly introduced into the pores of the porousoxide semiconductor film and the palladium compound is subsequentlysubjected to a reduction process to produce palladium particles in thepores.

Examples of palladium compounds that can be used for the purpose of theinvention include palladium acetate (Pd(CH₃COO) 2), palladium chloride(II) (PdCl₂), palladium nitrate (II) (Pd(NO₃)₂), dinitroamine palladium(II) ([Pd (NO₂)₂(NH₃)₂]), dichlorodiamine palladium (II)([Pd(NH₃)₂Cl₂]), tetraamine palladium (II) dichloride([Pd(NH₃)₄]Cl₂.nH₂O) and tetraamine palladium (II) nitrate(Pd(NH₃)₄(NO₃)₂), any of which may be used so long as it can producepalladium particles in the pores by way of the above-described process.

If, for example, platinum is introduced, a solution containing aplatinum compound is firstly introduced into the pores of the porousoxide semiconductor film and the platinum compound is subsequentlysubjected to a reduction process to produce platinum particles in thepores.

Examples of platinum compounds include chloroplatinic (IV) acid(H₂[PtCl₆].6H₂O), dinitrodiamineplatinum (II) ([Pt(NO₂)₂(NH₃)₂]),tetraaminedichloroplatinum (II) ([Pt(NH₃)₄]Cl₂.H₂O), potassiumhexahydroxoplatinate (IV) (K₂[Pt(OH)₆]) and platinum nitrate (IV)(Pt(NO₃)₄), any of which may be used so long as it can produce platinumparticles in the pores by way of the above-described process.

When particles of an oxide are to be held in the pores of the porousoxide semiconductor film, it is preferable to firstly form metalparticles, subsequently introduce the metal particles into the pores ofthe porous oxide semiconductor film and then oxidize the metal particlesin the pores to produce oxide particles.

For example, if particles of copper oxide are to be held in the pores,copper (Cu) particles are formed first and then the porous oxidesemiconductor film is immersed into a solution containing copperparticles in a dispersed state. Subsequently, the copper particles aresubjected to an oxidation process to produce copper (II) oxide (CuO) inthe pores.

As described above, it is possible to form a porous oxide semiconductorfilm that is provided with mesopores having a uniform pore size andmicrocrystal arranged in the pore walls and holding particles of aninorganic material in the pores as a result of carrying out Steps Athrough E.

Besides, it is possible to form a porous oxide semiconductor film whosesurfaces are modified by an organic material or an inorganic material asa result of carrying out Steps A through D (D1, D2).

Then, it is possible to prepare a sensor by connecting electrodes to theporous oxide semiconductor film.

For example, when the electrodes and the porous oxide semiconductor filmare to be connected to each other in an arrangement as shown in FIG. 2,the porous material and the electrodes can be connected well to eachother by firstly forming the electrodes on a substrate and subsequentlycarrying put the Steps A through D.

In the case of an arrangement as shown in FIG. 7, a porous oxidesemiconductor film is formed on a substrate first and then electrodesare formed thereon.

In the case of an arrangement as shown in FIG. 8, electrodes are formedon the substrate first and then a porous oxide semiconductor film isformed thereon. Alternatively, a porous oxide semiconductor film isformed on a substrate made of an electrode material and then anelectrode is formed on the porous oxide semiconductor film.

The process of forming electrodes may be conducted before or/and afterSteps A through D for forming a porous oxide semiconductor film.

When an oxide material is used, preferably it is a material that doesnot give rise to any chemical change due to an etching operation thatmay be conducted in the course of forming a porous oxide semiconductorfilm. More specifically, the use of gold (Au) or platinum (Pt) ispreferable.

However, any electrode material may be used for the purpose of thepresent invention if it does not give rise to any change in terms ofprofile and physical, chemical and/or electric properties thereof in thecourse of forming a porous oxide semiconductor film.

Techniques that can be used for forming electrodes for the purpose ofthe present invention include vacuum evaporation, sputtering,electro-deposition and other popular metal electrode forming methods.

EXAMPLES Example 1 An Example where the Pore Surfaces are Coated with anOrganic Material: an Organic Material Containing Aminopropyl Groups

In this example, a gas sensor element was prepared by forming a tinoxide porous thin film on a substrate that carries comb-shapedelectrodes formed thereon for the purpose of selectively detectingcarbon monoxide (CO).

Firstly, comb-shaped electrodes of platinum (Pt) were formed on a quartzsubstrate by photolithography in such a way that they were separatedfrom each other by a distance of 20 μm and have a length of 370 mm.

Then, 2.9 g of tin (II) chloride anhydride was added to 10 g of ethanol,which was then agitated for 30 minutes. Then, 1.0 g of tri-blockcopolymer P123 <HO(CH₂CH₂O)₂₀ (CH₂CH(CH₃)O)₇₀(CH₂CH₂O)₂₀H> was dissolvedin the ethanol, which was then agitated for another 30 minutes toproduce a precursor solution A.

Thereafter, the precursor solution A was applied to the comb-shapedelectrodes of the substrate by dip-coating.

Then, the substrate to which the precursor solution A was applied wasmoved into an environmental testing equipment and held in it.

The temperature and the relative humidity in the environmental testingequipment were controlled in a manner as described below.

The inside was held to 40° C. and 20% RH for 10 hours→The temperatureand the relative humidity were caused to change over 1 hour→The insidewas held to 50° C. and 90% RH for 5 hours→The temperature and therelative humidity were caused to change over 1 hour→The inside wasbrought back to 40° C. and 20% RH.

As a result of the above-described steps, a thin film of asurfactant-tin oxide meso structure was obtained.

Thereafter, the substrate was taken out from the environmental testingequipment and put into a muffle furnace and the temperature was raisedto 300° C. in air at a rate of 1° C./min and held to that temperaturefor 5 hours to remove the surfactant and obtain a mesoporous tin oxidethin film.

After the removal of the surfactant, it was confirmed by infraredspectrophotometry and the like that no organic material attributable tothe surfactant existed on the thin film.

When the surface and cross section of the thin film were observedthrough an SEM, tube-shaped structures were observed on the surface.

The cross section showed fine pores arranged like a honeycomb.

As a result of X-ray diffraction analysis, a clear diffraction peak thatcorresponds to an interplanar spacing of 4.9 nm was observed and adiffraction pattern that suggests a two-dimensional hexagonal structurewas obtained.

However, as a result of the observation of the cross section through anSEM, it was found that the hexagonal structure was shrunk in thedirection of the height of the film and hence it was not an idealhexagonal structure.

As a result of measuring the extent of nitrogen gas adsorption, it wasfound that the sizes of the fine pores showed a simple dispersion with amaximum value of 5.2 nm and that the distribution curve was found withina range not smaller than 1 nm and not greater than 10 nm.

The specific surface area was about 170 m²/g.

Thus, it was confirmed that the thin film was a porous thin film thathad substantially uniform mesopores and a large specific surface area.

As a result of an X-ray diffractmetric analysis using obliquely incidentX-rays, a clear peak ascribable to Cassiterite was confirmed.

The average crystal size L was determined to be equal to 2.7 nm by usingthe Scheller's formula shown below from the half breadth B (rad) and thepeak position 2θ of the peak attributable to the (211) plane in theregion of 2θ=45°−58° out of the appeared peaks.

L=0.9λ/B cos θ

From the above-described facts, it was confirmed that it is possible toform a porous tin oxide thin film having a pore structure of meso regionthat shows regularity and microcrystals in the pore walls on anelectrode substrate.

Then, the prepared porous tin oxide thin film was immersed in 10 g ofethanol solution with 0.5 g of 3-aminopropyltrimethoxysilane, which is asilane coupling agent.

Thereafter, the porous tin oxide thin film was washed in a flow of purewater and finally subjected to a dry process at 100° C. for 5 hours tomodify the surfaces of the fine pores by means of an organic materialcontaining aminopropyl groups.

In order to confirm the modification of the surfaces of the fine poresby the organic material, another porous tin oxide thin film was preparedon a high resistance silicon substrate by way of the above-describedsteps and the surfaces of the fine pores thereof were modified by anorganic material containing aminopropyl groups. Then, the thin film wassubjected to a measurement of infrared absorption spectrum.

As a result, the presence of an absorption spectrum attributable toamino groups was confirmed in the vicinity of 3,310-3,500 cm⁻¹.

For the purpose of comparison, a tin oxide thin film prepared withoutusing a surfactant and hence having no mesopores was subjected to amodification process and the obtained thin film was subjected to ameasurement of FT-IR absorption spectrum.

As a result of comparing the spectrums of the two specimens, the onehaving mesopores was confirmed to show a much stronger absorptioneffect. Thus, it was confirmed that the tin oxide thin film havingmesopores was modified to a larger extent.

Then, 7.7 mg of 3,4-dihydroxybenzoic acid was dissolved into 10 g ofwater, to which 9.3 mg of carbodiimide was added and the aqueoussolution was agitated.

Thereafter, the tin oxide thin film having mesopores, whose surfaceswere treated by the silane coupling agent, was immersed in the solutionfor 24 hours.

Subsequently, the tin oxide thin film was washed in a flow of pure waterand finally dried at 100° C. for 1 hour.

In order to confirm the modification by the organic material, thetreated sample was subjected to a measurement of infrared absorptionspectrum to find that an absorption spectrum attributable to hydroxygroups appeared in the vicinity of 3,450 cm⁻¹.

Such an absorption spectrum was not observed from the tin oxide thinfilm that was only treated by a silane coupling agent. Thus, it wasconfirmed that the 3,4-dihydroxybenzoic acid having hydroxy groupsreacted with the silane coupling agent and the reaction product wasformed on the surfaces of the fine pores.

Then, a gas sensor was prepared by connecting the electrode substrate onwhich the tin oxide porous thin film was formed to an electric circuitand the sensor characteristics of the gas sensor relative to mixed gaswere measured by means of a measuring apparatus as illustrated in FIG.11.

Three different types of mixed gas were used for the measurement. Theyinclude a mixed gas of air and carbon monoxide (CO), a mixed gas of airand methane (CH₄) and a mixed gas of air and hydrogen (H₂).

The concentration of each mixed gas could be adjusted by changing themixing ratio of air and the gas to be detected. In this example, theconcentration of each mixed gas was adjusted to 1,000 ppm and 500 ppmfor measurement. The measurement was conducted in a flow system underthe atmospheric pressure.

The method used for the measurements will be described below.

Only air was allowed to flow for 10 minutes→mixed gas with concentrationof 1,000 ppm was allowed to flow for 20 minutes→only air was allowed toflow for 20 minutes→mixed gas with concentration of 500 ppm was allowedto flow for 20 minutes→only air was allowed to flow.

A DC current of 1V was applied between the electrodes of the sensorelement to observe the electric current, while flowing air and mixed gasin the above-described order, and the observed electric current wasreduced to resistivity.

The temperature of the element was held to 100° C. during themeasurement.

FIG. 12 is a graph illustrating the change with time of the resistivityrelative to each of the mixed gases when measured under theabove-described conditions.

From FIG. 12, it was confirmed that the gas sensor element comprising atin oxide porous thin film, the surfaces of the fine pores of which weremodified by an organic material, of this example showed a raisedresistivity only to the mixed gas of CO and air so that it selectivelydetect CO.

If the resistivity of the gas sensor before the introduction of the COmixed gas is Ra, the smallest value of the resistivity after theintroduction of the CO mixed gas is Rco and the sensitivity Sco of thegas sensor element is expressed by the formula below, S=6 when the COmixed gas of a concentration of 1,000 ppm was allowed to flow and S=2.5when the CO mixed gas of a concentration of 500 ppm was allowed to flow.

Sco=Rco/Ra

From the above-described results, it was confirmed in this example thatit is possible to prepare a metal oxide semiconductor type gas sensorelement that shows selectivity relative to a specific gas and can highlysensitively detect the specific gas by using a tin oxide porous thinfilm, the surfaces of the fine pores of which are modified by an organicmaterial.

Comparative Example 1

FIG. 13 shows some of the results obtained by conducting a similarmeasurement on a gas sensor prepared by means of the method described inExample 1 except that tri-block copolymer P123 was not added.

The prepared gas sensor element selectively reacted to CO and air butthe changing rate of the electric resistivity was low if compared withthe gas sensor element prepared and used for the measurement inExample 1. More specifically, the sensitivity of the gas sensor elementwas S=1.1 for the CO mixed gas of a concentration of 1,000 ppm andS=1.05 for the CO mixed gas of a concentration of 500 ppm.

The changing rate of the electric resistivity was also low when H₂ mixedgas or CH₄ mixed gas was introduced if compared with the gas sensorelement of Example 1.

Example 2 An Example where the Pore Surfaces are Coated with an OrganicMaterial: an Organic Material Containing Aminopropyl Groups

In this example, a mesoporous tin oxide thin film was prepared as inExample 1 except that 0.7 g of tri-block copolymer F127<HO(CH₂CH₂O)₁₀₆(CH₂CH(CH₃)O)₇₀(CH₂CHO)₁₀₆OH> was used instead of 1.0 gof tri-block copolymer P123 <HO(CH₂CH₂O)₂₀(CH₂CH(CH₃)O)₇₀(CH₂CH₂O)₂₀H>when preparing the precursor solution.

When the surface and cross section of the thin film were observedthrough an SEM, the surface showed a structure where fine pores arearranged regularly and the fine pores were shrunk in the direction ofthe height of the film.

The cross section also showed fine pores arranged regularly.

As a result of X-ray diffraction analysis, a clear diffraction peak thatcorresponds to an interplanar spacing of 6.2 nm was observed.

Thus, it is safe to say that the tin oxide porous thin film is a cubicstructure having a large number of openings on the surface and showsregularity.

As a result of measuring the extent of nitrogen gas adsorption, it wasfound that the sizes of the fine pores showed a simple dispersion with amaximum value of 6.5 nm and that the distribution curve was found withina range not smaller than 2 nm and not greater than 12 nm.

The specific surface area was about 200 m²/g.

Thus, it was confirmed that the thin film was a porous thin film thathad substantially uniform mesoppres and a large specific surface area.

The average crystal size L was determined to be equal to 2.7 nm by usingthe Scheller's formula as in Example 1.

From the above-described facts, it was confirmed that it is possible toform a porous tin oxide thin film having a pore structure of the mesoregion that shows regularity and microcrystals in the pore walls on anelectrode substrate.

Then, the surfaces of the fine pores of the mesoporous tin oxide thinfilm were modified by an organic material as in Example 1 and the sensorcharacteristics of the gas sensor of this example relative to mixedgases were measured by using the same apparatus and method as those ofExample 1. Then, it was found that the changing rate with time of theelectric resistivity relative to each of the mixed gases wassubstantially same as its counterpart of Example as shown in FIG. 12.

From the above-described results, it was confirmed in this example thatit is possible to prepare a metal oxide semiconductor type gas sensorelement that shows selectivity relative to a specific gas and can highlysensitively detect the specific gas by using a tin oxide porous thinfilm, the surfaces of the fine pores of which are modified by an organicmaterial.

Example 3 An Example where the Pore Surfaces are Coated with anInorganic Material: Silicon Oxide

In this example, a gas sensor element was prepared by forming a tinoxide porous thin film on a substrate on which a pair of comb-shapedelectrodes had been formed and used for selectively detecting H₂ gas.

Firstly a mesoporous tin oxide thin film was formed on a substrate, onwhich a pair of comb-shaped Pt electrodes had been formed, by means of amethod similar to the one used in Example 2. It was confirmed that aporous tin oxide thin film having a pore structure of the meso regionthat shows regularity and microcrystals in the pore walls on anelectrode substrate was formed as in Example 1.

Then, the prepared porous tin oxide thin film was immersed in a toluenesolution showing a diethoxydimethylsilane (DEMS) concentration of 1 wt %for about 10 minutes.

Subsequently, the porous tin oxide thin film subjected to the aboveprocess was washed in a flow of pure water and finally subjected to adry process at 100° C. for 5 hours to modify the surfaces of the finepores by means of an organic material containing silicon.

Thereafter, the porous tin oxide thin film was baked at 300° C. for 5hours.

In order to confirm the modification of the surfaces of the fine poresby silicon oxide, another porous tin oxide thin film was prepared on ahigh resistance silicon substrate by way of the above-described stepsand the prepared porous tin oxide thin film was observed by means of aTEM.

For the purpose of comparison, a specimen that had not been immersed inthe DEMS solution was also observed by means of a TEM.

As a result, it was confirmed that the both specimens had fine poresthat were arranged regularly, they were different from each other interms of the contrast in the surface areas of the fine pores and that,as a result of the observation by means of TEM-DES, the presence of Sion and near the surfaces of the fine pores was confirmed on the specimenthat had been subjected to a DEMS immersion process.

The adsorption isotherm of each of the samples was determined and thedistribution of fine pore sizes was computationally determined by theBerret-Joyner-Halenda (BJH) method to confirm that the fine pore sizedistribution of the specimen that had been subjected to a DEMS immersionprocess was found in smaller fine pore sizes.

From the above findings, it was confirmed that silicon oxide was formedin the fine pores of the porous tin oxide thin film, while the poroustin oxide thin film was maintaining the fine pore structure.

Subsequently, the sensor characteristics of the gas sensor of thisexample relative to mixed gases were measured by using the sameapparatus and method as those of Example 1.

The temperature of the element was held to 150° C. during themeasurement. FIG. 14 illustrates some of the obtained results.

The sensor was highly responsive relative to H₂ mixed gas if comparedwith its responsiveness relative to any other mixed gas. If theresistivity of the gas sensor before the introduction of H₂ mixed gas isRa, the smallest value of the resistivity after the introduction of H₂mixed gas is RH₂ and the sensitivity SH₂ of the gas sensor element isexpressed by the formula below, S=300 when the H₂ mixed gas of aconcentration of 1,000 ppm was allowed to flow and S=160 when the H₂mixed gas of a concentration of 500 ppm was allowed to flow.

SH₂=Ra/RH₂

The sensitivity of the gas sensor relative to each of the remainingmixed gases was computationally determined by using a similar formula,it was found that Sco=40 when the CO mixed gas of a concentration of1,000 ppm was allowed to flow and SCH₄=3 when the Cl₄ mixed gas of aconcentration of 1,000 ppm was allowed to flow.

From the above-described results, it was confirmed in this example thatit is possible to prepare a metal oxide semiconductor type gas sensorelement that shows selectivity relative to a specific H₂ gas and canhighly sensitively detect the specific gas by using a tin oxide porousthin film that contains micro-crystals in the pore walls, and thesurfaces of the fine pores of which are modified by an organic material.

Comparative. Example 2

FIG. 15 shows some of the results obtained by conducting a similarmeasurement on a gas sensor prepared by means of the method described inExample 3 except that tri-block copolymer F127 was not added.

For instance, SH₂=30 for the H₂ mixed gas of a concentration of 1,000ppm and Sco=20 for the CO mixed gas of a concentration of 1,000 ppm,whereas SCH₄ 1.5 for the CH₄ mixed gas of a concentration of 1,000 ppm.Thus, the sensitivity to any of the mixed gases was low if compared withthe gas sensor element prepared in Example 3. The sensitivity wasparticularly low relative to H₂ mixed gas.

Example 4 An Example where Particles of an Inorganic Material were Heldin the Pores: Metal Palladium

In this example, a gas sensor element was prepared by forming a tinoxide porous thin film on a substrate on which a pair of comb-shapedelectrodes had been formed and used for selectively detecting hydrogen(H₂) gas.

Firstly, comb-shaped electrodes of platinum (Pt) were formed on a quartzsubstrate by photolithography in such a way that they were separatedfrom each other by a distance of 20 μm and have a length of 370 mm.

Then, 2.9 g of tin (II) chloride anhydride was added to 10 g of ethanol,which was then agitated for 30 minutes. Then, 1.0 g of tri-blockcopolymer P123 <HO(CH₂CH₂O)₂₀ (CH₂CH(CH₃)O)₇₀(CH₂CH₂O)₂₀H> was dissolvedin the ethanol, which was then agitated for another 30 minutes toproduce a precursor solution A.

Thereafter, the precursor solution A was applied to the comb-shapedelectrodes of the substrate by dip-coating.

Then, the substrate to which the precursor solution A was applied wasmoved into an environmental testing equipment and held in it. Thetemperature and the relative humidity in the environmental testingequipment were controlled in a manner as described below.

The inside was firstly held to 40° C. and 20% RH for 10 hours.Thereafter, the temperature and the relative humidity were caused tochange over 1 hour to 50° C. and 90% RH and then held constant for 5hours. Subsequently, the temperature and the relative humidity werebrought back respectively to 40° C. and 20% RH to obtain asurfactant-tin oxide meso structure material.

Thereafter, the substrate was taken out from the environmental testingequipment and put into a muffle furnace and the temperature was raisedto 300° C. in air at a rate of 1° C./min and held to that temperaturefor 5 hours to obtain a mesoporous tin oxide thin film.

When the surface and cross section of the film were observed through ascanning electron microscope (SEM), tube-shaped structures were observedon the surface.

The cross section showed fine pores arranged like a honeycomb.

As a result of X-ray diffraction analysis, a clear diffraction peak thatcorresponds to an interplanar spacing of 4.9 nm was observed and adiffraction pattern that suggests a two-dimensional hexagonal structurewas obtained.

However, as a result of the observation Of the cross section through anSEM, it was found that the hexagonal structure was shrunk in thedirection of the height of the film and hence it was not an idealhexagonal structure.

As a result of measuring the extent of nitrogen gas adsorption, it wasfound that the sizes of the fine pores showed a simple dispersion with amaximum value of 5.2 nm and that the distribution curve was found withina range not smaller than 1 nm and not greater than 10 nm.

The specific surface area was about 170 m²/g.

Thus, it was confirmed that the film was a porous film that hadsubstantially uniform mesopores and a large specific surface area.

As a result of an X-ray diffractmetric analysis using obliquely incidentX-rays to the film, a clear peak ascribable to Cassiterite wasconfirmed.

The average crystal size L was determined to be equal to 2.7 nm by usingthe Scheller's formula shown below from the half breadth B (rad) and thepeak position 2θ of the peak attributable to the (211) plane in theregion of 2θ=45°−58° out of the appeared peaks.

L=0.9λ/B cos θ

From the above-described facts, it was confirmed that it is possible toform a porous tin oxide thin film having a pore structure of the mesoregion that shows regularity and microcrystals in the pore walls on anelectrode substrate.

Then, the prepared porous tin oxide thin film was immersed in an aqueoussolution of ammonium where the concentration of palladium acetate(Pd(CH₃COO)₂) was 0.005M.

Subsequently, after a drying process, the solution was heated at 300° C.for 1 hour in a hydrogen atmosphere and then at 300° C. for 5 hours inthe air and subjected to a reduction process again at 300° C. in ahydrogen atmosphere in order to obtain metal palladium particles.

In order to confirm that metal palladium particles were held in thepores, the structure of the surface and that of the cross section of thethin film were observed through a transmission electron microscope (TEM)

As a result, it was confirmed that particles of a diameter of about 3 nmwere held in the pores. When the particles were analyzed by means of anenergy dispersion type X-ray spectrometer (EDS) and an electron energyloss spectrometer (EELS) annexed to the TEM to confirm that theparticles held in the pores were those of metal palladium.

As a result of X-ray diffraction analysis, a diffraction pattern thatsuggests a two-dimensional hexagonal structure was obtained. Thus, itwas confirmed that the pore structure was maintained after causingpalladium particles to be held in the pores.

As a result of measuring the extent of nitrogen gas adsorption, it wasfound that the specific surface area was about 180 m²/g and hence hadnot changed significantly after causing palladium particles to be heldin the pores.

Then, the electrode substrate where the tin oxide porous film was formedwas connected to an electric circuit and the sensor characteristics ofthe gas sensor of this example relative to mixed gas were measured bymeans of a measuring apparatus as illustrated in FIG. 11.

Three different types of mixed gases were used for the measurement. Theyinclude a mixed gas of air and hydrogen (H₂) (to be referred to as gas Ahereinafter), a mixed gas of air and methane (CH₄) (to be referred to asgas B hereinafter) and a mixed gas of air and nitrogen monoxide (NO) (tobe referred to as gas C hereinafter).

The concentration of each mixed gas could be adjusted by changing themixing ratio of air and the gas to be detected. In this example, theconcentration of each mixed gas was adjusted to 200 ppm, 100 ppm and 50ppm to measure the change in the resistivity relative to the gasconcentrations.

The measurement was conducted in a flow system under the atmosphericpressure. Each of the gases was introduced with each of the above-citedconcentrations during a gas On period and only air was introduced duringa gas Oft period.

A DC current of 1V was applied between the electrodes of the sensor toobserve the electric current, while flowing each of the above citedmixed gases, and the change with time of the electric current wasobserved. The temperature of the sensor was held to 100° C. during themeasurement.

FIG. 16 is a graph illustrating the change with time of the resistivityrelative to each of the mixed gases as determined from the change withtime of the electric current.

It was found that the gas sensor of this example comprising a tin oxideporous film selectively responded to gas A, or mixed gas of air andhydrogen. In other words, it scarcely responded to gas B and gas C.

As for the change with time of the resistivity relative to gas A, if theresistivity observed in a gas Off period when only air was allowed toflow is Ra, the resistivity observed in a gas On; period when the mixedgas of air and hydrogen gas was allowed to flow is RH and thesensitivity SE of the gas sensor is defined by the formula below, it wasfound that SH=100 relative to hydrogen with the concentration of 200ppm, SH=40 relative to hydrogen with the concentration of 100 ppm andSH=20 relative to hydrogen with the concentration of 50 ppm.

SH=Ra/RH

From the above-described results, it was confirmed in this example thatit is possible to prepare a metal oxide semiconductor type gas sensorthat shows selectivity relative to a specific gas and can highlysensitively detect the specific gas by using a tin oxide porous thinfilm that contains microcrystals on the pore walls and holds metalpalladium particles in the pores.

Comparative Example 3

FIG. 17 shows some of the results obtained by conducting a similarmeasurement on a gas sensor prepared by means of the method described inExample 4 except that no surfactant was added.

It reacted slightly to gas A but scarcely to gas B and gas C.

As for gas A, it was found that SH=10 relative to hydrogen with theconcentration of 200 ppm, SH=4 relative to hydrogen with theconcentration of 100 ppm and SH=1.5 relative to hydrogen with theconcentration of 50 ppm.

Example 5 An Example where Particles of an Inorganic Material were Heldin the Pores Platinum

In this example, a gas sensor element was prepared by forming a tinoxide porous thin film on a substrate on which a pair of comb-shapedelectrodes had been formed and used for selectively detecting H₂ gas.

Electrodes and a porous ton oxide film were prepared on a quartzsubstrate as in Example 4.

Then, the prepared porous tin oxide film was immersed in an aqueoussolution of platinic chloride (H₂PtCl₆), where the concentration ofplatinic chloride was 0.005M.

Thereafter, the porous tin oxide film was subjected to a drying processand then to a reduction process at 170° C. for 2 hours in a hydrogenatmosphere in order to obtain platinum particles.

In order to confirm that platinum particles were held in the pores, thestructure of the surface and that of a cross section of the thin filmwere observed through a transmission electron microscope (TEM).

As a result, it was confirmed that particles of a diameter of about 3 nmwere held in the pores.

When the particles were analyzed by means of an energy dispersion typeX-ray spectrometer (EDS) and an electron energy loss spectrometer (EELS)annexed to the TEM to confirm that the particles held in the pores werethose of metal platinum.

As a result of X-ray diffraction analysis, a diffraction pattern thatsuggests a two-dimensional hexagonal structure was obtained. Thus, itwas confirmed that the pore structure was maintained after causingplatinum particles to be held in the pores.

As a result of measuring the extent of nitrogen gas adsorption, it wasfound that the specific surface area was about 180 m₂/g and hence hadnot changed significantly after causing palladium particles to be heldin the pores.

The sensor characteristics of the gas sensor of this example relative tomixed gases were observed by means of an apparatus and a method similarto those of Example 4 to find that the change with time of theresistivity to each of the gases of the gas sensor of this example wassimilar to the one illustrated in FIG. 16.

For the purpose of comparison, a similar gas sensor was prepared exceptthat no surfactant was added and subjected to a similar observation tofind that the change with time of the resistivity of this gas sensor wassimilar to the one illustrated in FIG. 17.

From the above-described results, it was confirmed in this example thatit is possible to prepare a metal oxide semiconductor type gas sensorelement that shows selectivity relative to a specific gas and can highlysensitively detect the specific gas by using a tin oxide porous filmthat contains microcrystals on the pore walls and holds platinumparticles in the pores.

Example 6 An Example where Particles of an Inorganic Material were Heldin the Pores: Copper (II) Oxide

In this example, a gas sensor element was prepared by forming a tinoxide porous film on a substrate on which a pair of comb-shapedelectrodes had been formed and used for selectively detecting hydrogensulfide (H₂S) gas.

Electrodes and a porous tin oxide film were prepared as in Example 4.

Then, the prepared porous tin oxide film was immersed in an aqueoussolution in which copper (Cu) particles prepared by laser abrasion weredispersed and subjected to a supersonic process for about 1 hour. Then,it was heated at 300° C. for 1 hour in the air so as to introduceparticles of copper (II) oxide (CuO) into the pores.

In order to confirm, that particles of copper (II) oxide (CuO) were heldin the pores, the structure of the surface and that of the cross sectionof the thin film were observed through a transmission electronmicroscope (TEM).

As a result, it was confirmed that particles of a diameter of about 3 nmwere held in the pores.

When the particles were analyzed by means of an energy dispersion typeX-ray spectrometer (EDS) and an electron energy loss spectrometer (EELS)annexed to the TEM to confirm that the particles held in the pores werethose of copper (II) oxide.

As a result of X-ray diffraction analysis, a diffraction pattern thatsuggests a two-dimensional hexagonal structure was obtained. Thus, itwas confirmed that the pore structure was maintained after causingparticles of copper (II) oxide to be held in the pores.

As a result of measuring the extent of nitrogen gas adsorption, it wasfound that the specific surface area was about 180 m₂/g and hence hadnot changed significantly after causing copper (II) oxide particles tobe held in the pores.

The sensor characteristics of the gas sensor of this example relative tomixed gases were observed by means of an apparatus and a method similarto those of Example 4, But, mixed gas of air and hydrogen sulfide (H2S)was applied as gas A instead of mixed gas of air and hydrogen (H2) inthis example. Consequently, the change with time of the resistivity toeach of the gases of the gas sensor of this example was similar to theone illustrated in FIG. 16.

For the purpose of comparison, a similar gas sensor was prepared exceptthat no surfactant was added and subjected to a similar observation tofind that the change with time of the resistivity of this gas sensor wassimilar to the one illustrated in FIG. 17.

From the above-described results, it was confirmed in this example thatit is possible to prepare a metal oxide semiconductor type gas sensorelement that shows selectivity relative to a specific gas and can highlysensitively detect the specific gas by using a tin oxide porous thinfilm that contains microcrystals on the pore walls and holds particlesof copper (II) oxide in the pores.

A sensor and a method of manufacturing the same according to theinvention can be applied to a gas sensor for selectively detecting aparticular type of gas. Additionally, a sensor according to the presentinvention can find applications in the field of gas sensors fordetecting gas and bio sensors for detecting bio substances.

Furthermore, a sensor and a method of manufacturing the same accordingto the invention can find applications in the field of gas sensors forselectively detecting a particular type of gas.

This application claims priority from Japanese Patent Application Nos.2004-329046 filed on Nov. 12, 2004, 2004-376368 filed on Dec. 27, 2004and filed on Jun. 7, 2005, which are hereby incorporated by referenceherein.

1-10. (canceled)
 11. A method of manufacturing a sensor comprising thesteps of: preparing a reaction solution containing a metal compound anda surfactant to apply the reaction solution onto a substrate; holdingthe substrate with the reaction solution applied thereto in asteam-containing atmosphere to form a film containing a metal oxide andthe surfactant on the substrate; removing the surfactant from the filmto produce a film having a plurality of mesopores; and causing themesopores to hold particles of an inorganic material.
 12. A method ofmanufacturing a sensor comprising the steps of: preparing a reactionsolution containing a metal compound and a surfactant to apply thereaction solution onto a substrate; holding the substrate with thereaction solution applied thereto in a steam-containing atmosphere toform a film containing a metal oxide and the surfactant on thesubstrate; removing the surfactant from the film to produce a filmhaving a plurality of mesopores; and coating at least part of surfacesin the mesopores with an organic material.
 13. A method of manufacturinga sensor comprising the steps of: preparing a reaction solutioncontaining a metal compound and a surfactant to apply the reactionsolution onto a substrate; holding the substrate with the reactionsolution applied thereto in a steam-containing atmosphere to form a filmcontaining a metal oxide and the surfactant on the substrate; removingthe surfactant from the film to produce a film having a plurality ofmesopores; and coating at least part of surfaces in the mesopores withan oxide.
 14. The method according to claim 11, wherein the metalcompound is a chloride containing a metal, an alkoxide containing ametal or an isopropoxide containing a metal.
 15. The method according toclaim 11, wherein the surfactant contains ethylene oxide as ahydrophilic group.
 16. The method according to claim 11, wherein thestep of causing the mesopores to hold particles of an inorganic materialincludes a step of introducing an inorganic compound or an inorganicmaterial into the mosopores and a step of forming particles of aninorganic material or an inorganic oxide in the mesopores by oxidizingor reducing the inorganic compound or the inorganic material.
 17. Themethod according to claim 12, wherein the step of coating at least partof surfaces in the mesopores with an organic material is a step ofbonding the organic material to the surfaces by means of a silanecoupling agent. 18-19. (canceled)